Direct detect sensor for flat panel displays

ABSTRACT

Each sensor of a linear array of sensors includes, in part, a sensing electrode and an associated feedback circuit. The sensing electrodes are adapted to be brought in proximity to a flat panel having formed thereon a multitude of pixel electrodes in order to capacitively measure the voltage of the pixel electrodes. Each feedback circuit is adapted to actively drive its associated electrode via a feedback signal so as to maintain the voltage of its associated electrode at a substantially fixed bias. Each feedback circuit may include an amplifier having a first input terminal coupled to the sensing electrode and a second input terminal coupled to receive a biasing voltage. The output signal of the amplification circuit is used to generate the feedback signal that actively drives the sensing electrode. The biasing voltage may be the ground potential.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 USC 119(e) of thefollowing U.S. provisional applications, the contents of all of whichare incorporated herein by reference in their entirety: Application No.60/673,967, Attorney Docket No. 014116-009600US, filed Apr. 22, 2005,entitled “Detector For Measuring Functionality Of LCD Flat-PanelPixels;” Application No. 60/687,621, Attorney Docket No.014116-010400US, filed Jun. 2, 2005, entitled “Testing Of LCDElectrode;” Application No. 60/689,601, Attorney Docket No.014116-010500US, filed Jun. 9, 2005, entitled “Testing Of LCDElectrode;” Application No. 60/697,844, Attorney Docket No.014116-010600US, filed Jul. 8, 2005, entitled “Direct Detect Sensor ForOLED Display.”

BACKGROUND OF THE INVENTION

In a finished Liquid Crystal Flat Panel, a thin layer of liquid crystal(LC) material is disposed between two sheets of glass. On one sheet ofglass, a two-dimensional array of electrodes has been patterned. Eachelectrode may be on the order of 100 microns in size and can have aunique voltage applied to it via multiplexing transistors positionedalong the edge of the panel. In a finished product, the electric fieldcreated by each individual electrode couples into the LC material andmodulates the amount of transmitted light in that pixelated region. Thiseffect when taken in aggregate across the entire 2-D array results in avisible image on the flat-panel.

A significant part of the manufacturing cost associated with LCD panelsoccurs when the LC material is injected between the upper and lowerglass plates. It is therefore important to identify and correct anyimage quality problems prior to this manufacturing step. The problemwith inspecting LCD panels prior to deposition of the liquid crystal(LC) material is that without LC material, there is no visible imageavailable to inspect. Prior to deposition of LC material, the onlysignal present at a given pixel is the electric field generated by thevoltage on that pixel (assuming no physical contact is made with thepixels).

To overcome this limitation, Photon Dynamics developed a floatingmodulator which, in part, includes a relatively large piece of opticallyflat glass with a thin layer of LC material formed on its surface, asshown in FIG. 1A.

To inspect the patterned glass plate 10, modulator 15 is physicallymoved over a region 20 to be inspected and then lowered to within a fewmicrons of the flat-panel's surface, as shown in FIG. 1B. The small airgap 25 between the flat-panel electrodes 30 and the LC modulator 15allows the electric field from each pixel electrode 30 on the patternedglass plate 10 to couple to modulator 15 to create a temporary visibledisplay of the panel. This visible display is subsequently captured bycamera 35 for identification of defects. After inspecting region 20,modulator 15 is lifted and moved to another region on the panel and theprocess is repeated. Through this step-and-repeat process, the entire LCpanel can be inspected for defects. As shown in FIGS. 1A and 1B, LCmodulator 15 is shown as including, an LC material 45 and a flat glass50.

There is a growing need to increase the inspection speed. Inspecting anLCD panel at high speeds using the modulator described above posestechnical challenges. For example, the need to physically lift themodulator (which may weigh several pounds) from its present site, moveit to the next site and then lower it in preparation for the nextinspection operation affects the system throughput.

Moreover, with the modulator described above, the visible image createdon the thin LCD layer is obtained by reflecting light from the surfaceof the LC material. The LC material acts a scattering medium in itsoff-state and a transmissive medium in the on-state. This typicallyresults in the generation of a DC-component of light modulated with arelatively small mount of information. To camera 35, this means that theimager must be able to handle a relatively large signal (for the DCcomponent) even though the signal containing the information isrelatively weak. Furthermore, the relatively large DC-component of lightcomponent may carry a correspondingly large amount of shot noise whichneeds to be overcome to enable one to reproduce the flat-panel defectdata. Furthermore, presently known modulators do not readily lendthemselves directly to a continuous, linear scanning.

Non-contact capacitive coupling techniques have been developed to testLCD flat panel arrays. In accordance with one such known method, anelectrically floating (open-circuited) conductive plate or a diffusionregion is brought into close proximity of the LCD panel. This causes thevoltage on the LCD pixel to capacitively couple to the floating plate,thereby causing its voltage to vary in proportion to the ratio of theair-gap capacitance to the parasitic capacitances (plate to substrate aswell as plate to surrounding circuitry). This voltage change can then bebuffered and supplied off-chip to be measured. FIG. 2 shows atwo-dimensional array 60 of sensors that may be capacitively coupled totest an LCD panel. Such two-dimensional arrays 60 suffer from a numberof disadvantages.

First, such two-dimensional arrays require step-and-repeat movements,thus lowering the testing throughput. Second, the parasitic capacitancesof such arrays are relatively large which may result in poorsensitivity. Furthermore, since many of the parasitic capacitances arenon-linear (especially when diffusions regions are used) the sensoritself behaves nonlinearly. Moreover, in such two-dimensional arrays,the read-out addressing lines which select which pixel values are sentoff-chip, have relatively larger parasitic capacitances.

As is shown in FIG. 2, array 60 is adapted to include both horizontaladdress lines X and vertical address lines Y running through each pixelelement. The distance between these addressing lines and the floatingplate will typically be less than the distance from the detector chip tothe LCD panel. Therefore, the amount of addressing crosstalk seen in theoutput data is often relatively large. Furthermore, when testing, e.g.,a 40 microns×40 microns per pixel element using two-dimensional array60, it is required that the sensing circuitry and the sensing electrode(floating plate) for each pixel fit within substantially the same, e.g.,40×40 microns² area. The area limitation imposed by the horizontal andvertical dimensions of any given pixel prevents the development and useof complex sensing circuitry on two-dimensional arrays. Accordingly, thetwo-dimensional arrays are forced to use simple sensing circuitry thatmay not be effective.

FIG. 3 shows a passive conductive plate 205 positioned in closeproximity of an LCD pixel electrode 210 to sense the voltage on LCDpixel electrode 210, as known in the prior art. The LCD pixel electrode210 and the opposing passive electrode 205 form a simple parallel platecapacitor having a capacitance defined by εA/D, where ε is thedielectric constant of the material between the plates, A is the platearea and D is the separation distance between the plates. The degree towhich the LCD voltage is coupled to the opposing electrode is determinedby the ratio of the parallel plate capacitance defined by plates 205,210, to the other parasitic capacitances, such as C1 and C2, amongothers. The larger these parasitic capacitances, the smaller the size ofthe coupled voltage. Moreover, many of the parasitic capacitance arenon-linear and result in a non-linear response in the couplingcharacteristic. With the exception of reset transistor 230 whichperiodically resets the passive electrode 205 to a known DC levelVreset, electrode 205 is floating or passive during the sensing process.As is shown, transistor 230 has a source terminal coupled to sensingelectrode 205, a gate terminal receiving reset clock signal reset_clk,and a drain terminal coupled to the reset biasing voltage Vreset.

While simple to implement, there are numerous disadvantages to the priorart sensing technique shown in FIG. 3. First, since the passiveelectrode 205 is floating, its voltage changes as the LCD pixel voltagecoupled thereto via plate 210 varies. Accordingly, the parasiticcapacitance C1 and C2 directly impact the coupling sensitivity. Second,since the load resistor 220 is adapted to provide gain, the apparentcapacitance seen at C1, which is the parasitic capacitance between thegate and drain of transistor 215, is multiplied by this gain, due to thewell-known Miller gain effect. This will further reduce detectionsensitivity. Third, any noise on the power supply Vdd will coupledirectly into the output signal Output. Fourth, any variations in thegain of transistor 215 due to aging, temperature or processing directlyimpacts the output signal quality. Fifth, the multiplication effect ofthe capacitance C1 reduces the bandwidth of the gain stage provided bytransistor 215. Sixth, since the parasitic capacitances arepredominantly junction capacitances, the voltage coupling is non-linear.Seventh, the circuit output is limited to a binary logic state, anddetection depends on a time-varying change in voltage in the LCD pixelelements during the sensing process.

Active matrix organic light emitting diode (AMOLED) displays requirebackplanes made with either amorphous or polycrystalline-silicon thinfilm transistors (TFT). Polycrystalline silicon displays requirefabrication using low temperature processes (LTPS) in order to avoiddamage to glass and especially flexible (e.g. plastic) substrates. Thefabrication of AMOLED backplanes using LTPS can be quite complexrequiring as many as, for example, 10 mask steps with precision controlrequirements. This has been identified as a potential challenge for lowcost, high yield manufacturing of large scale AMOLED displays. Thefabrication of AMOLED displays using amorphous Si backplanes may requirefewer mask steps, but is nearly as challenging. As AMOLED displaysbecome larger, the need for inspection and yield management becomes morecritical. Efforts are underway to improve these processes. However,there has been less focus on the development of AMOLED inspection tools,even though they offer the dual promise of more efficient convergence onprocess development as well as improved yield and lowered cost in AMOLEDmanufacturing—by capturing killer defects early in the fabricationcycle. As AMOLED displays grow in size and value for the monitor and TVmarkets, the need for inspections tools will become critical.

One conventional method of inspecting OLED display is to opticallyinspect the backplanes. FIG. 4 shows an x-y array 400 of pads adapted toreceive OLEDs 402. Each OLED 402 _(ij) pad is coupled via an associatedtransistor 420 _(ij) to a data line and to a gate line, where index irefers to the row and index j refers to the column in which the OLED pad402 _(ij) is disposed. Three such data lines, 406, 408, and 410 areshown, and four such gate lines 412, 414, 416, and 418 are shown. Forexample, OLED pad 402 ₁₁ is shown as being coupled to data line 406 andto gate line 416 via transistor 420 ₁₁. Optical testing does not providefunctional information on the pixels.

BRIEF SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, each sensorof a linear array of sensors includes, in part, a sensing plate(electrode) and an associated feedback circuit. The sensing electrodesare adapted to be brought in proximity to a flat panel having formedthereon a multitude of pixel electrodes so as to measure the voltage ofthe pixel electrodes capacitively, i.e., in a non-contact manner. Eachfeedback circuit is adapted to actively drive its associated sensingelectrode so as to maintain the voltage of its associated sensingelectrode at a substantially fixed bias. The feedback circuits enablesensing of the pixel voltages without requiring temporal variations inthe pixel voltages. The linear array of sensors is adapted to be scannedover the panel at a constant scanning rate. The flat panel may includeLCD pixels, OLED pixels, or the like.

In one embodiment, each feedback circuit includes, in part, anamplification circuit having a first input terminal coupled to thesensing electrode and a second input terminal coupled to receive abiasing voltage. The output signal of the amplification circuit is usedto generate the feedback signal that actively drives the sensingelectrode. The feedback circuit and its associated sensing electrode maybe formed on the same semiconductor substrate or on differentsemiconductor substrates.

In one embodiment, the biasing voltage supply is the ground potential,however, it is understood that any other DC biasing voltage may be used.In one embodiment the amplification circuit is an operational amplifier(op-amp), however, it is understood that any other amplificationcircuit, notwithstanding its complexity, which uses feedback to maintaina fixed voltage at the amplifier input may be used. In such embodiments,a capacitive elements may be coupled between the first input terminaland the output terminal of the amplifier. The output signal of theop-amp changes in linear proportion to the pixel electrode voltage.Since the op-amp actively drives its associated sensing electrode to aknown DC potential, parasitic capacitances at the input terminal ofop-amp have a relatively small effect on the detection sensitivity.

To perform the testing, a fixed pattern of DC voltages is applied to thepixels on the panel at the beginning of the scan. As the linear array ismoved across the board and each new pixel is scanned, the feedbackcircuit associated with each sensing electrode receives the signalsensed thereby as a result of capacitive coupling. The amount of currentrequired to maintain each sensing electrode at the substantially fixedbiasing voltage is integrated on the feedback capacitor and provides ameasure of the sensed electric field generated by the pixel electrodeand capacitively coupled to that sensing electrode. The op-amp may bereset periodically to avoid drifts caused by leakage currents.

In one embodiment, the LCD panel is periodically refreshed to inhibitdrooping of the pixel voltages. Because in the present invention, thelinear scanning is continuous, during the refresh period some of thescanned pixel data may not be valid. To ensure that every row of pixelsis scanned during a period when the LCD panel data is valid, a secondlinear array of sensors positioned at a known distance away from thefirst linear array of sensors is used. Thus the pixel rows that arescanned by the first linear array during the periods when data isinvalid are scanned by the second linear array after the refresh iscomplete and the data is again valid.

Defects such as weak shorts or leaking transistors are detected bymeasuring the amount of the voltage droop on a pixel after the elapse ofa known time period following a refresh cycle. To accomplish this, athird linear array of sensors spaced away from the first and secondarray of sensors is used. The voltage droops are measured by a pair oflinear sensors at two different instances of time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a floating modulator positioned above a patterned glassplate, as known in the prior art.

FIG. 1B shows the floating modulator of FIG. 1 positioned in proximityof the patterned glass plate to perform testing, as known in the priorart.

FIG. 2 shows a two-dimensional array of sensors, as known in the priorart.

FIG. 3 shows a passive conductive plate positioned in close proximity toan LCD pixel electrode to sense the voltage thereon, as known in theprior art.

FIG. 4 shows a two-dimensional array of pads adapted to receive OLEDs,as known in the prior art.

FIG. 5 shows a sensor positioned in proximity of a pixel electrode totest the voltage present thereon, in accordance with one embodiment ofthe present invention.

FIG. 6A shows an actively driven sensor, in accordance with oneembodiment of the present invention.

FIG. 6B shows an actively driven sensor, in accordance with anotherembodiment of the present invention.

FIG. 7A shows a linear array of actively driven sensors, in accordancewith one embodiment of the present invention.

FIG. 7B shows a simplified cross-sectional view of the linear array ofthe sensors shown in FIG. 7A.

FIG. 7C shows a linear array of actively driven sensors, in accordancewith another embodiment of the present invention.

FIG. 8 shows a multitude of linear array sensors, in accordance with oneembodiment of the present invention.

FIG. 9 shows a CCD sensor adapted to be used in the linear sensor arrayof the present invention, in accordance with one embodiment of thepresent invention.

FIG. 10 shows a multitude of actively driven sensors disposed in alinear sensor array, in accordance with one embodiment of the presentinvention.

FIG. 11 shows a cross-sectional view of a linear array of sensors, inaccordance with one embodiment of the present invention.

FIG. 12 shows an array of OLEDs adapted to be test using an array oflinear sensors, in accordance with one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with one embodiment of the present invention, a lineararray of sensors is brought in proximity (e.g., 10 microns to 100microns) to a flat panel under test, and the electric field generated bythe panel electrodes is capacitively measured. In one exemplaryembodiment, the flat panel is an LCD panel and each sensor includes asensing electrode and an amplifying circuit. A scanning rate of about100 millimeters per second or about 100 microns per millisecond may beused. The following description is provided with reference to an LCDpanel. It is understood, however, that the present invention is equallyapplicable to any other types of panels.

FIG. 5 shows a sensor 300 positioned in proximity to panel 320 whichincludes a multitude of pixels 330, only one of which is shown. Sensor300 is shown as including a sensing electrode (conductive plate) 305 aswell as an amplification circuit 310, in accordance with one embodimentof the present invention. FIGS. 6A shows sensor 300 in whichamplification circuit 310 is an operational amplifier (op-amp) beingused in a feed-back configuration. In the exemplary embodiment shown inFIG. 6A, the negative input terminal of op-amp 310 is connected tosensing electrode 305 and to the output terminal of the op-amp 310 viafeedback capacitor 315. The positive input terminal of op-amp 310 isshown as connected to the ground terminal. Sensing electrode 305 isactively driven to maintain its voltage at the known voltage of theground potential. Consequently, because the positive input terminal ofop-amp 310 receives the ground potential, as the sensor 300 is broughtinto proximity of an LCD pixel, sensing electrode 305 is maintained atthe virtual ground potential, notwithstanding the voltage of the pixelbeing sensed. In other words, sensing electrode 305 is maintained at thevirtual ground potential regardless of the opposing pixel voltage beingsensed.

FIG. 6B is a schematic diagram of a sensor 300, in accordance withanother embodiment of the present invention. Sensing electrode 305 isactively driven to a fixed DC level through the use of the high-gainfeedback network. The displacement charge necessary to maintain thisvoltage is sensed as the LCD pixel 365 is brought into close proximityof sensing electrode 305. The exemplary feedback network is shown asincluding an op-amp 310, and a capacitor 315. Op-amp 310 is adapted tomaintain the voltage on sensing electrode 305 to the bias voltage VBiasapplied to the non-inverting input terminal 380 of op-amp 310. It isunderstood that the present invention is applicable to any otherfeedback circuitry configured to sense and actively drive sensorelectrode 305 to maintain its voltage at a substantially constant value.For example, in one embodiment, sensor 300 has an open loop gain ofnearly 1000. As a consequence, in such embodiments, the voltage changesat sensing electrode 305 are kept to 1/1000^(th) of the voltage changesseen at the output of the amplifier 310. Accordingly, if for example,the maximum voltage swing at the amplifier output is about 2 volts, thevoltage at sensing electrode 305 changes by a maximum of 2 mV.

In the exemplary embodiment shown in FIGS. 6A and 6B, because of thefeedback, the current flows from the inverting side of the op-amp toinsure that the voltage on the electrode 305 does not vary as the LCDpixel electrode is brought into its close proximity. The high-impedanceof the op-amp input terminals requires that substantially most of thecurrent which flows to the sensing electrode 305 come from the feedbackcapacitor 315. The resulting change in voltage across the feedbackcapacitor 315 is defined by Q/C, where Q is the total charge needed tomaintain the sense electrode 305 at the fixed potential and C is thecapacitance of capacitor 315.

The output signal of op-amp 310 changes in linear proportion to the LCDpixel electrode voltage, where the constant of proportionality is C.Consequently, the sensor array can distinguish pixel outputs of varyinggray scale values with fixed DC level voltage applied to the pixels.Since op-amp 310 actively drives sensing electrode 305 to a known DCpotential, parasitic capacitances at the input terminals of op-amp 310have a relatively small effect on the detection sensitivity. Otheradvantages of the feedback configuration, in accordance with the presentinvention, include, among other things, (i) power supply noiserejection, (ii) linearity since the gain mechanism is controlled by thelinear feedback capacitor rather than a non-linear open-loop transistorcharacteristic, (iii) immunity to gain, processing differences and agingvariation due to the feedback approach, and (iv) wide bandwidth due tothe fact that the Miller gain multiplier is substantially eliminated.

It is understood that the sensing circuit, which in the exemplaryembodiments of FIGS. 6A and 6B is shown as including an op-amp 310 and acapacitor 315, defining a feed-back network, and sensing electrode 305may be formed on the same semiconductor substrate. Alternatively theop-amp 310 and capacitor 305 may be formed on a semiconductor substratedifferent from the one on which sensing electrode 305 is formed.

To perform the testing, a fixed pattern of DC voltages is applied to thepixels on the panel (board) at the beginning of the scan. The currentrequired by op-amp 310 to keep sensing electrode 305 at the constantpotential provides a measure of the pixel voltage. As the linear arrayis moved across the board and each new pixel is scanned, op-amp 310senses the resultant electric field of the static pattern. The op-ampmay be reset periodically, e.g. at 30 Hz or less, to avoid drifts causedby any leakage current.

FIG. 7A is a top view 500 of M linear arrays (rows) 505 ₁, 505 ₂ . . .505 _(M) of sensing electrodes, in accordance with one embodiment of thepresent invention. Because the sensor arrays of the present inventionare linear, only row decoding is required to access any of the sensingelectrodes—no column decoding is required. In other words, to access anyof the sensors disposed in any of the rows 505 ₁, 505 ₂ . . . 505 _(M)only row address decoding is performed. Top view 500 of the linearsensor elements is shown as having N sensing electrodes 510 _(ij) ineach of the M rows, where index i refers to the row and index j refersto the column in which the sensing electrode is disposed. For example,row 505 ₁ is shown as including sensing electrodes 510 ₁₁, 510 ₁₂ . . .510 _(1N), and row 505 _(M) is shown as including sensing electrodes 510_(M1), 510 _(M2) . . . 510 _(MN). Formed below each of the sensingelectrodes 510 _(ij) is the sensing circuitry associated with thatsensing electrode. FIG. 7B shows a simplified cross-sectional view oflinear array 5051. As is seen from FIG. 7B, each sensing circuitry 520_(ij) is positioned below its associated sensing electrode 510 _(ij) towhich it is coupled.

FIG. 7C is a top view 700 of M linear arrays (rows) 705 ₁, 705 ₂ . . .705 _(M) of sensing electrodes, in accordance with one embodiment of thepresent invention. In accordance with embodiment 700, sensing circuits720 _(ij) are disposed between sensing electrodes 710 _(ij) in a checkerboard format. In FIG. 7C, sensing circuits 720 _(ij) are shown withinshaded regions to indicate that they are formed within the siliconsubstrate and adjacent the sensing electrodes.

Because the sensors of the present invention are arranged linearly, anumber of advantages, such as advantages in physical placement of thesensors as well as the detection circuitry, are achieved over the priorart two dimensional arrays are. For example, if each sensor has an xdimension of 40 microns, and a y dimension of 40 microns, each sensingelectrode 305 is formed to be of the same size, e.g., 40 microns×40microns thus enabling the associated electronic circuitry, e.g.,amplifier, capacitor, and the like, to be formed adjacent the sensingelectrode. The additional silicon real-estate available below eachsensing electrode enables the formation of more complex circuitry, suchas op-amps 310 which use feedback to hold the pixel plate to a knownvalue, as described above, and shown in FIGS. 6A and 6B.

The use of feedback, as shown in FIGS. 6A and 6B, enables amplifier 310to have a highly linear response and to collect repeatable data that isless dependent on fabrication processes. The feedback arrangement viacapacitor 315 also minimizes temperature variations and aging effectsduring operation. This feedback maintains the voltage on sensingelectrode 305 to a known value regardless of the LCD pixel valueundergoing sensing and measurement. Thus, instead of sensing the LCDvoltage change directly, each sensor of the present invention measuresthe amount of current required by the op-amp 310 to maintain sensingelectrode 305 at a constant potential for differing LCD pixel voltages.Since the voltage on sensing electrode 305 does not change, sensitivityis not degraded by parasitic capacitance. Moreover, since there is onlyneed for addressing in one dimension, addressing lines may be runoutside of the area of the active sensing electrode, thus eliminatingnoise generated by addressing feed-through.

When using a shorting bar prober, the limited bandwidth of an LCD panelallows for changing a given LCD pixel voltage about 60 times per second.It is therefore crucial that the sensor be adapted so as not to requirea temporal change in the LCD pixel voltage in order to sense each newpixel value. Conventional prior art sensing techniques require atemporal change in the LCD pixel voltage of at least 2 KHz in order toenable the sensing be performed, this is a difficult rate to achieve. Asensor, in accordance with the present invention, senses the LCD pixelvoltage without requiring temporal variations in the LCD pixel voltages.To achieve this, at the beginning of a scan, a fixed pattern of voltagesis applied across the array of the LCD panel. As the linear array ofsensors is moved across the array and each new LCD pixel is scanned,op-amp 310 senses the resultant electric field of the static pattern. Insome embodiments, op-amp 310 may need to be reset periodically, e.g. 30Hz, to avoid drift from leakage currents.

In one embodiment, the LCD panel is refreshed once every, e.g., 30-50ms, to inhibit the voltages applied to the pixels from drooping overtime. This refreshing process may require, e.g., about 4-7 ms tocomplete. Because in the present invention the linear scanning iscontinuous, during the refresh period some of the scanned LCD panel datamay not be valid. To overcome this, in some embodiments, in part, atleast two linear sensors separated by a known physical distance aredisposed on the same chip. The use of a pair of sensors ensures that thedata missed by the leading sensor array during the LCD panel refreshinterval is picked up by the second sensor which follows the firstsensor during the scan process. In other words, to ensure that every rowof pixels is scanned during a period when the LCD panel data is valid, asecond array is spaced from the first so that the second array passesover a given row of pixels about, e.g., 7-10 ms (somewhat more than thetime of invalid data) after the first array. Thus those rows that arescanned by the first array during the periods when data is invalid arescanned by the second array after the refresh is complete and the datais again valid.

FIG. 8 shows a multitude of linear array sensors 600, in accordance withone embodiment of the present invention. Only four of the linear arrays,namely arrays 605, 610, 615, and 620 are shown. During testing, sensorarray 600 is caused to travel along the YY′ direction. The distancebetween arrays 605 and 615 is D2 microns, and similarly the distancebetween arrays 610 and 620 is D2 microns. At a scan rate of 100 micronsper ms, D2=700-1000 microns. Although not shown, it is understood thatmany more linear array sensors with similar characteristics and havingcorresponding physical relationships may be used.

Assume that linear array sensors 600 are traveling along YY′ direction.The linear arrays are so disposed such that pixels with invalid datasensed by the leading sensors in array 620 during a refresh cycle of alinear scan are sensed and their voltages captured by sensor 610 afterelapse of a known time period. Similarly, pixels with invalid datasensed by the sensors in leading array 615 during such a refresh cycleof the linear scan, are sensed and their voltages captured by sensor 605during that scan.

The present invention is also adapted to detect the presence of suchdefects, as weak shorts or leaking transistors. These types of defectsare measured by observing the voltage on a pixel some known time afterthe refresh cycle to see how much droop has occurred in the voltagelevel. To accomplish this, associated with each array is a second arrayspaced D1 microns from that array. Referring to FIG. 8, arrays 605 and610 are spaced D1 microns apart, e.g., about 1 to 5 ms delayed duringthe scan. At a scan rate of 100 microns per ms, D1=100 to 500 microns.Similarly, arrays 615 and 620 are also D1 microns apart. The voltagedroops are measured in sensor 600 by the corresponding array pairs thatare spaced D1 microns apart. For example, linear arrays 605 and 610 areused collectively to detect the amount of voltage droop experienced bythe pixels being tested.

In some embodiment, the linear array sensor uses a CCD architecture inthe form of fill-and-spill samplers coupled to the floating gates, asshown in FIG. 9. Each CCD control gate 810 in the array of sensorelements is in electrical communication with an associated metal plate830 which, in some embodiments, is roughly ½ the narrow dimension of theLCD pixel undergoing measurement. In other words, in such embodiments,if the LCD pixel is 100 microns wide and 300 microns tall, the CCD'smetal plate (sensing electrode) may be 50 microns wide and 50 micronstall. To sense the e-field on the LCD panel, the array of metal plates830 (only one of which is shown) is be brought in close proximity (10 to100 microns) of the LCD panel electrodes 820 SO as to enable theelectric field on the LCD pixels to capacitively couple to metal plates830. Coupling of the electric field to the CCD metal plates induces avoltage in the CCD fill-and-spill sampler which is converted into acharge packet representation. Once in this form, the signal can be readout in a manner similar to a normal CCD imager.

Some embodiments of the present invention include N linear sensor arraysconfigured to concurrently measure the voltages on different LCD pixels.To accommodate transfer of the data retrieved from any one of the arraysat any given time, one or more multiplexers are used. FIG. 10 shows anLCD panel 10 positioned in proximity to N linear array of sensors 300 ₁,300 ₂ . . . 300 _(N). The signals sensed by the sensors are received bymultiplexer 805 which selectively supplies one of the received signalsat its output. The voltages supplied by each of the op-amps 310 is equalto the voltage of the pixel sensed by that op-amp 3 10 multiplied by theratio C₁/C₂, where C₁ is the capacitance of any one of capacitors 315,and C₂ is the capacitance defined by any one of the sensing electrodes305 and the opposing pixel electrode.

FIG. 11 shows a linear array 900 of sensors, in accordance with oneembodiment of the present invention. In embodiment 900, capacitorsensing electrodes 905 are formed on glass substrate 920, and the activesensing circuitry, including the sense amps, etc., are formed on asemiconductor substrate 930. The array includes N capacitor plates 905₁, 905 ₂ . . . 905 _(N), formed on glass substrate 920, and N activesensing circuits 910 ₁, 910 ₂ . . . 910 _(N), formed on semiconductorsubstrate 920. Semiconductor substrate 930 and glass substrate 920 areinsulated from one another via an insulating dielectric, such as silicondioxide or silicon nitride. It is understood that the capacitor platesand active sensing circuits identified with the same index, areassociated with and coupled to one another. For example, capacitor plate905, is coupled to and is associated with active sensing circuit 910 ₁via metal 915 ₁. In some embodiments, spin-on glass may be used to formboth the glass substrate, on which capacitor plates 905 are formed, aswell as the insulating dielectric. In such embodiments, the glasssubstrate layer and the insulating dielectric layer are parts of thesame layer that is formed using the spin-on glass.

A linear array sensor, in accordance with some embodiment of the presentinvention, is operative to test OLED panels. To achieve this, a testline is disposed in each row of OLED pads to enable the transistorsdisposed in such rows to be tested using the direct detect sensingdescribed above. FIG. 12 shows an x-y array 950 of pads adapted toreceive OLEDs 902. Each OLED 902 _(ij) pad is coupled via an associatedtransistor 904 _(ij) to a data line and to a gate line, where index irefers to the row and index j refers to the column in which the OLED padis disposed. Three such data lines, 940 ₁, 940 ₂, 940 ₃, and four suchgate lines 925 ₁, 925 ₂, 925 ₃, and 925 _(M) are shown. For example,OLED pad 902 ₁₁ is shown as being coupled to data line 940 ₁ and gateline 925 ₁ via transistor 904 ₁₁. It is understood that array 950typically has more data and gate lines than that shown in FIG. 12.

Also disposed in each row is a test line positioned in parallel to thegate line. For example test line 930 ₁ is disposed in parallel to andadjacent gate line 925 ₁; test line 930 ₂ is disposed in parallel to andadjacent gate line 925 ₂, etc. In some embodiments, each test line isformed by a metal layer coupled to either the source or the drainterminal of the transistors disposed in that row. For example, test line930 ₁ is coupled to either the source or the drain terminals A, B and Cof transistors 904 ₁₁, 904 ₁₂ and 904 ₁₃. The other drain/sourceterminals of these transistors is coupled to data lines 940 ₁, 940 ₂ and940 ₃. In the embodiment shown in FIG. 12, each test line is shown asbeing coupled to a resistor. For example, test line 930 ₁ is coupled toone of the terminals of resistor 960.

When a voltage is applied to any of the gate lines, all the transistorscoupled to that row are turned on. For example, when gate line 925 ₂receives a high voltage, transistors 904 ₂₁, 904 ₂₂ and 904 ₂₃ areturned on. This causes a current to flow from the source (drain)terminals to the drain (source) terminals when a voltage potential iscaused to appear across these terminals. The current flow causes apotential to develop at nodes E, F and G, respectively, of pads 902 ₂₁,902 ₂₂ and 902 ₂₃. The voltages developed at the pads 902 _(ij) may besubsequently measured in accordance with the direct detect sensingtechnique of the present invention to identify defects. Therefore, byallowing current to flow, the current carrying capacity of the pixeltransistors could be characterized using the direct detect sensing (DDS)or other voltage sensing technology.

The direct detect sensing of the present invention identifies defectsand provides process control data during the fabrication of OLEDbackplanes when the backplane is modified. The DDS together with a pixelload transistor enables current characterization on OLED backplane.Furthermore, combination of DDS with differential measurement ofadjacent OLED pixels with (or without) pixel load transistor enablesdetection of small pixel current (or voltage) defects. Therefore, inaccordance with the present invention, defects are detected and processcontrol in OLED roll-to-roll fabrication is achieved.

In some embodiments of the present invention, instead of a test lineconnecting the output of the pixel transistor to a remote load resistor,the backplane could be modified to include a test transistor that wouldroute a test current directly to ground when active. In yet otherembodiments, the DDS is adapted to detect only the differentials betweenadjacent test pixels. This may be implemented either as a softwareprogram or in hardware by sampling the data stream at two pointsseparated in time or space by an amount that corresponds to theseparation between OLED pixels. The resulting signals would then besubtracted and defect detection algorithms would then be applied to thisdifferential signal. This differential system may be vulnerable tovoltage noise but the high SNR values of the DDS system is well suitedfor such applications.

Testing of the OLEDs, in accordance with present invention provides anumber of advantages. Among such advantages are as follows. The testingin accordance with the present invention is faster and has a greater S/Nthan e-beam inspection tools; TACTs consistent with in-line operation(i.e., ˜60 seconds/plate). The invention may be operated in anyenvironment (from vacuum to atmospheric pressures with any degree ofhumidity control). The invention has spatial resolution on the scale of10's of μm. The invention is scalable to large formats. The invention isadaptable to flexible substrate. The invention may also be used tomeasure current.

The above embodiments of the present invention are illustrative and notlimiting. Various alternatives and equivalents are possible. Theinvention is not limited by the type of amplifier or amplificationcircuitry, feedback circuitry, biasing voltage, etc., used in thesensing circuits. The invention is not limited by the number of lineararrays nor is it limited by the number of sensors disposed in each linerarray. The invention is not limited by the scanning rate. The inventionis not limited by the type of integrated circuit in which the presentdisclosure may be disposed. Nor is the disclosure limited to anyspecific type of process technology, e.g., CMOS, Bipolar, or BICMOS thatmay be used to manufacture the present disclosure. Other additions,subtractions or modifications are obvious in view of the presentdisclosure and are intended to fall within the scope of the appendedclaims.

1-8. (canceled)
 9. A method of testing a panel having formed thereon aplurality of pixels, the method comprising: capacitively coupling asensing electrode to a pixel electrode at time T1 to sense the pixelelectrode voltage; and maintaining said sensing electrode pixel at asubstantially constant voltage via a feedback signal generated inaccordance with the sensed pixel electrode voltage, wherein said sensingelectrode is disposed in a linear of array of sensors.
 10. The method ofclaim 9 further comprising: supplying the sensed pixel electrode voltageto a first input terminal of an amplifying circuit; and generating thefeedback signal from an output voltage generated by the amplifyingcircuit.
 11. The method of claim 10 wherein said amplifying circuitcomprises an operational amplifier.
 12. The method of claim 10 furthercomprising: supplying a biasing voltage to a second input terminal ofthe amplifying circuit.
 13. The method of claim 12 wherein said biasingvoltage is the ground potential.
 14. The method of claim 12 furthercomprising: capacitively coupling the output terminal of the amplifyingcircuit to the first input terminal of the amplifying circuit.
 15. Themethod of claim 10 further comprising: capacitively coupling a secondsensing electrode to the pixel electrode at time T2 to sense the pixelelectrode voltage; wherein T2 and T1 are spaced in time by a predefinedvalue; and maintaining said second sensing electrode pixel at asubstantially constant voltage via a feedback signal generated inaccordance with the pixel electrode voltage sensed by the second sensedelectrode.
 16. The method of claim 15 further comprising: capacitivelycoupling a third sensing electrode to the pixel electrode at time T3 tosense the pixel electrode voltage; wherein T3 and T1 are spaced in timeby a predefined value; and maintaining said third sensing electrodepixel at a substantially constant voltage via a feedback signalgenerated in accordance with the pixel electrode voltage sensed by thethird sensed electrode.
 17. The method of claim 16 wherein each of thesecond and third sensors further comprises: a sensing electrode adaptedto be capacitively coupled to the pixel electrode disposed on the panel;and an associated feedback network configured to maintain the voltage ofthe sensing electrode associated therewith at a substantially constantvoltage when positioned in proximity of the pixel electrode to becapacitively coupled thereto.
 18. The method of claim 9 wherein saidpixel electrode receives a DC voltage before being capacitively coupledto the sensing electrode.